1. Field of the Invention
The subject invention relates to an antenna, specifically a multi-element antenna in an array-type configuration, for receiving a circularly polarized radio frequency (RF) signal from a satellite and a linearly polarized RF signal from a terrestrial source.
2. Description of the Prior Art
Vehicles have long implemented glass to enclose a cabin of the vehicle while still allowing visibility for the driver of the vehicle. Automotive glass is typically either a tempered (or toughened) glass or a laminated glass which is produced by bonding two or more panes of glass together with a plastic interlayer. The interlayer keeps the panes of glass together even when the glass is broken.
Recently, antennas have been integrated with the glass of the vehicle. This integration helps improve the aerodynamic performance of the vehicle as well to help provide the vehicle with an aesthetically-pleasing, streamlined appearance. Integration of antennas for receiving linearly polarized RF signals, such as those generated by AM/FM terrestrial broadcast stations, has been the principal focus of the industry.
However, that focus is shifting to integrating antennas for receiving RF signals from Satellite Digital Audio Radio Service (SDARS) providers. SDARS providers use satellites to broadcast RF signals, particularly circularly polarized RF signals, back to Earth. SDARS providers use multiple satellites in a geostationary orbit or in an inclined elliptical constellation. The elevation angle between the respective satellite and the antenna is variable depending on the location of the satellite and the location of the antenna. Within the continental United States, this elevation angle may be as low as 20°. Accordingly, specifications of the SDARS providers require a relatively high gain at elevation angles as low as 20°. SDARS providers also use terrestrial “repeater” stations to rebroadcast their satellite signal. These terrestrial stations operate at an elevation angle of 0° and are useful in urban environments where tall buildings may obstruct signals from the satellites. Linear polarization is used for these terrestrial rebroadcasts.
Additionally, automotive manufacturers and vehicle drivers demand that the antenna integrated with the glass does not obstruct the view of the driver. Therefore, it is typically a requirement that the antenna occupy less than a certain surface area, or “footprint”, when integrated with the glass.
Various antennas for receiving both circularly polarized and linearly polarized RF signals are known in the art. Examples of such antennas are disclosed in the U.S. Pat. No. 6,697,019 (the '019 patent) to Hyuk-Joon et al and U.S. Pat. No. 6,545,647 (the '647 patent) to Sievenpiper et al. The '019 patent discloses an antenna system installable on the roof of a vehicle for receiving RF signals produced by circularly polarized transmitters and linearly polarized transmitters. The antenna includes four linear polarized radiation elements and four circularly polarized radiation elements arranged symmetrically about a center. The antenna includes a circuit board for supporting the linear polarized radiation elements and a dielectric substrate. The linear polarized radiation elements each have a brick shape and include a microstrip resonator having a length of one-quarter wavelength λ. The circularly polarized radiation elements are microstrip patches disposed on the dielectric substrate. The circularly polarized radiation elements each have a square shape that is geometrically different from that of the linearly polarized radiation elements. The antenna system also includes a 90-degree hybrid. The 90-degree hybrid shifts the signal to two of the circularly polarized radiation elements by 90 degrees while the signal to the other two circularly polarized radiation elements is unshifted. The antenna requires separate feed lines for the linear and circular polarized signals.
Since the antenna of the '019 patent is a large, bulky array of antenna elements for mounting on the roof of the vehicle, it is not suitable for integration with a window of the vehicle. If the antenna of the '019 patent were to be mounted onto the window, the eight separate elements would occupy a large surface area and obstruct the view of a driver of the vehicle. Furthermore, the antenna does not significantly aid in reception of RF signals from low elevation angles.
The '647 patent discloses an antenna for receiving RF signals produced by circularly polarized transmitters and linearly polarized transmitters. The antenna includes four radiation elements arranged symmetrically about a center and disposed on a high impedance surface. The high impedance surface acts as a ground plane and is typically mounted on a large metallic object, such as a roof of a vehicle. The radiation elements are formed of an electrically conductive material and implemented either as pieces of wire or metallic patches. Various connections of phase-shift circuits to the radiation elements give the antenna its circular and linear polarizations. The antenna requires separate feed lines for a receiver to receive the linear and circular polarized signals. The antenna of the '647 patent does not significantly aid in reception of RF signals from low elevation angles.
There remains an opportunity to introduce an antenna that aids in the reception of the RF signal from a satellite. Particularly, there remains an opportunity for an antenna that aids in reception of the RF signal from elevation angles as low as 20°. Furthermore, there remains an opportunity for an antenna that does not significantly obstruct the view of the driver of the vehicle and provides both circular and linear polarized signals on a single feed line.